"We believe that we have reached the point where we can confidently state that interest in gold investing has reached the level of a speculative bubble," the bank's investment team wrote in a market update issued Aug. 15. "Prudent investors should be very wary of having substantial investment exposure to this precious metal in their portfolios."

Quickly and with very little warning, the bottom can drop out on gold prices, the report said. During a six-month period in 2008, gold lost more than 30 percent of its value. In the 1980s, in just over two years, the price plummeted about 65 percent.